With the development of electronic data transmission systems using large-scale integration techniques, a need arose to provide efficient and compatible frequency selective circuits for such systems. Conventional active filters using thin film or other hybrid technology, although an improvement over discrete component passive filters, failed to meet the requirements of LSI data system applications. Charge-transfer devices (CTD) were suggested as one approach for implementing analog sampled-data filters but were found to require a large plurality of stages to provide the necessary narrow transition from pass band to stop band and were therefore relatively inefficient in their use of silicon area. Also such CTD filters has an inherently high insertion loss, i.e. around 20 db.
Another more successful approach to the problem was the more recent development of so-called "switched capacitor" filters using MOS technology. (See MOS Sampled Data Recursive Filters Using Switched Capacitor Integrators, IEEE Journal of Solid-State Circuits, Vol. SC-12, No. 6, December 1977).
In the aforesaid article, a sampled data filter circuit is disclosed utilizing a pair of operational amplifiers in series, each being connected in parallel with a capacitor. Switches in the circuit, controlled by clock pules, alternate between their two positions to provide two different outputs. The reference circuit operates to realize a pair of complex poles in the transfer function for the circuit and resulted in a relatively low sensitivity filter. However, it failed to provide a filter with a sharp enough transition at the desired frequency. The present invention solves the aforesaid problem and provides an improved sampled data filter having several advantages including greater efficiency with a loss characteristic that closely approximates the more ideal elliptic filter.